The present invention relates to a jam detector for a transfer paper sheet.
A conventional jam detector has a configuration as shown in FIG. 1. Referring to FIG. 1, paper sheets P (to be referred to as transfer paper sheets hereinafter) stored in a cassette 5, which is disposed in a paper feed section 3 located at the right-hand side in FIG. 1, are sequentially fed out therefrom along a conveyance path 1 by means of a paper feed roller 7. The transfer paper sheet P is guided to a photosensitive drum 11 with an electrostatic latent image formed thereon through register rollers 9 which are disposed in the feed direction. After the latent image formed on the photosensitive drum 11 is transferred onto the transfer paper sheet P by chargers 13 (transfer and separation chargers), the transfer paper sheet P is discharged from a discharge section 15. A paper feed switch SW1 is arranged near the paper feed section 3 to detect the state of paper feeding. A discharging switch SW2 is arranged near the discharge section 15 to detect the state of paper discharge. The switches SW1 and SW2 are arranged mainly to detect paper jamming. In this example, after the transfer paper P is fed, the leading end of the transfer paper sheet P is aligned by the register rollers 9 and is then conveyed to the photosensitive drum 11. A predetermined time interval after the register rollers 9 start rotating, the discharging switch SW2 arranged in the discharge section 15 is checked to see if the transfer paper sheet P has been detected. If the discharging switch SW2 does not detect the transfer paper sheet P, it is determined that a jam has occurred between the register rollers 9 and the discharge section 15. Even if the leading end of the transfer paper sheet P is detected by the switches SW1 and SW2 at the proper time, when the trailing end of the transfer paper sheet P is detected by the switch SW1 but is not detected by the switch SW2 within a predetermined time interval thereafter, it is determined that a jam has occurred in the discharge switch area.
In the conventional detector of the above arrangement for detecting a jam after the predetermined conveying time of the transfer paper sheet P elapses when transfer paper sheets P are sequentially conveyed at predetermined intervals, a plurality of timers must be arranged corresponding to the number of transfer paper sheets. A complex control circuit is required for the switching and setting of timers, resulting in a high manufacturing cost, time-consuming operation and degradation of the reliability of the apparatus.